Four block copolymer melts-two poly(isoprene-bloch-dimethylsiloxane) (PI-PDMS) and two poly(ethylene-alt-propylene-block-dimethylsiloxane) (PEP-PDMS)-have been investigated through rheological measurements, SANS, and SAXS. In both systems, lamellar to disorder and bicontinuous to disorder transitions have been observed. Upon heating, the sequence of phases lamellae, modified layers, bicontinuous , disorder is observed in the f = 0.65 PEP-PDMS sample. Composition fluctuations dominate the disordered state properties over an unprecedented wide temperature window in the f = 0.49 PEP-PDMS sample. The fluctuations are evident both in SANS measurements and through the temperature dependence of the viscosity. The phase behavior of these relatively low molar mass block copolymer melts, which range from 6 to 10 kg/mol, is qualitatively anticipated by the trends reported earlier for progressively lower molar mass diblock copolymers.